The present disclosure relates in general to optical sensors, and more particularly to optical angle of arrival (AOA) sensors.
Conventional AOA sensors generally include an aperture that collects incident light, such as the light from a laser designator or laser illuminator that is back scattered from an object. The aperture projects an illuminated spot onto a 4-quadrant detector. The size of the illuminated spot is slightly larger than the size of one quadrant of the detector, so that at least two quadrants are illuminated. Each detector quadrant produces an output current. The values of the four output currents are then processed to determine the location of the center or the centroid of the illuminated spot and thus the angle of arrival of the incident light relative to the common axis of the aperture and detector.
Other sensors are known such as a dual-mode optical sensor that combines an AOA sensor (that is based on a silicon 4-quadrant detector) and a focal-plane-array (FPA) imaging detector. In one configuration of this sensor, the 4-quadrant detector is located between an optical imaging system (a set of lenses) and the focal plane array imaging detector. Thus, the light to be detected by the imager first passes through the silicon 4-quadrant detector. However, a 4-quadrant detector is most effective when the spot of light projected onto that detector is approximately equal to and slightly larger than the size of one of the quadrants. The centroid of the large spot illuminating the 4-quadrant detector is determined by comparing the relative amounts of optical energy detected by each of the four quadrants. Thus, any speckle in the optical illumination pattern at the lens or input aperture of this sensor and any occlusion of the input aperture produce a non-uniform optical intensity distribution across the projected spot and result in an erroneous estimate of the excursion of the corresponding centroid, and thus the angle of arrival of the light.
In some AOA sensors, instead of using a large defocused spot, the AOA sensor uses a small focused spot of light that is not affected or is minimally affected by speckle or by occlusions of the input aperture. For example, a lateral-effect position sensing detector (LEPSD) can sense the position of a small focused or nearly focused spot of light. However, in a conventional LEPSD, the maximum allowable excursion of the spot of light is limited by the size of the light-detecting area of that device. The LEPSD typically includes a set of two electrodes with one set of electrodes used to determine the x-location and the other set used to determine the y-location of the spot of light. The relative location of the incident spot of light with respect to the two electrodes of a set determines the relative amounts of the two currents that are output from those two electrodes.
Most two-axis LEPSD devices are fabricated with silicon detector material. Because silicon is an indirect-bandgap semiconductor material, the light-absorbing layer of a silicon photodetector must be very thick (typically 30-100 μm thick). Thus, the capacitance per unit area of silicon photodetectors is small. Because conventional LEPSD devices use silicon material, these devices are not sensitive to eye-safe laser wavelengths (wavelengths >1.4 μm). To sense the longer wavelengths, the LEPSD must be made from a direct-bandgap material such as InGaAs or InAs, InAsSb, InSb or HgCdTe. However, for example, the InGaAs LEPSD has a PIN diode structure with the two InP layers being the P-layer and the N-layer of the diode and the undoped or lightly doped InGaAs layer being the I-layer of the diode. Because the I-layer is so thin, with a typical thickness being 1-3 μm, the capacitance per unit area of an InGaAs LEPSD is quite high and can limit the response bandwidth and the rise time of a large-area device.
Some devices use multiple photodiode detectors that are configured to function like a single-axis LEPSD, wherein the collection of multiple photodiodes acts like the light-absorbing region of one LEPSD. The photodiodes are physically arranged in a linear array and are electrically connected to a linear chain of resistors, with a photodiode connected to each junction between two resistors. However, these devices can distinguish only as many distinct spatial locations for the incident light as the number of photodiodes in the chain. Thus, to achieve high resolution for an AOA sensor based on such a resistively combined array, a very large number of photodiodes is needed.
Some devices include two-dimensional arrays having elements that are LEPSDs rather than conventional photodiodes. For the arrays in these devices, the analog outputs from the multiple LEPSDs elements are provided on a shared set of output lines, but in a time-multiplexed manner, similar to the output for an imager array. Thus, the raster-scanned, time-multiplexed outputs for the multiple array elements are provided on a frame-by-frame basis. As a result, the response bandwidth of these arrays is limited to the frame rate, which generally is slower than 100 kHz. The slow response achieved by the frame-by-frame multiplexing allows these arrays to detect only the occurrences of pulses that are spaced far apart in time and cannot provide any information on the temporal shape of moderately narrow pulses of incident light. In some devices, each LEPSD is provided separately and the outputs for each LEPSD element are connected to separate analog-to-digital converters. The digital signals from the analog-to-digital converters are then processed together. Thus, these LEPSD arrays have the limitation that in order to achieve the large response bandwidth suitable for detecting and discerning short pulses or closely spaced pulses, the individual LEPSD elements must have outputs digitized separately.
Some known devices also include an array of LEPSD elements that is integrated with an image-detecting array to form a dual-band sensor that has approximately the same image plane for the wavelengths of light sensed by the AOA sensor and the wavelengths of light sensed by the image-detecting array. However, these dual-band sensors require a complicated achromatic optical imaging system to provide AOA operation. Unless complicated achromatic designs are used, the imaging system would define different focal-plane locations for the different wavelengths of light. Thus, conventional dual-band, dual-mode sensors that combine an AOA sensor and an image-detecting array and that use the same optical imaging system (e.g., lens) for both the AOA sensor and the image-detecting array either project a defocused spot of light onto the AOA sensor or require complicated achromatic designs for their optical imaging systems.